feat(connect): add --server-name flag for tunneled connections

[jimmystridh]

Problem

When connecting to Azure SQL through an SSH tunnel or proxy (e.g., 127.0.0.1:11433), connections fail because SQL Server validates the hostname in the TDS LOGIN7 packet. The server receives 127.0.0.1 as the server name and rejects it with "Cannot open server '127.0.0.1' requested by the login".

Current workaround requires adding /etc/hosts entries mapping the real server name to 127.0.0.1.

Fixes #576

Solution

Add a --server-name flag that specifies the server name to send in the login packet, separate from the dial address (-S).

# Connect via tunnel on localhost:11433, authenticate as the real server
sqlcmd -S 127.0.0.1,11433 --server-name myserver.database.windows.net \
    -U myuser -P mypass -N -C

Implementation

• Add ServerNameOverride field to ConnectSettings
• Build the connection string using the override host/instance (so LOGIN7 uses the override)
• Use a proxy dialer that rewrites the network dial target back to the original -S host/port
• Skip override for named pipes

[jimmystridh]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

Adds support for overriding the server name used during TDS login (and related connection metadata) so sqlcmd can connect through tunnels/proxies (e.g., 127.0.0.1:11433) while authenticating as the real Azure SQL hostname.

Changes:

• Introduces ConnectSettings.ServerNameOverride and applies it when building the connection string.
• Adds a proxyDialer to rewrite the actual dial target while keeping the overridden server name for login.
• Adds unit tests covering the new connection string behavior and dialer behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/sqlcmd/connect.go	Adds ServerNameOverride and applies it during connection string construction.
pkg/sqlcmd/sqlcmd.go	Attempts to attach a custom dialer when ServerNameOverride is set.
pkg/sqlcmd/dialer.go	Implements a dialer that rewrites dial host/port while reporting an overridden hostname.
pkg/sqlcmd/dialer_test.go	Adds unit tests for proxyDialer.
pkg/sqlcmd/sqlcmd_test.go	Extends connection string tests to cover ServerNameOverride.

[Copilot]

connector is now typed as *mssql.Connector, but the AAD path calls GetTokenBasedConnection(...) which currently returns driver.Connector (see pkg/sqlcmd/azure_auth.go). This will cause a type mismatch/compile error and also makes it hard to support ServerNameOverride for AAD connectors. Consider keeping connector as driver.Connector and only type-asserting to *mssql.Connector when setting Dialer (or update GetTokenBasedConnection to return *mssql.Connector consistently).

[Copilot]

PR description/title mention adding a --server-name flag, but the repo currently has no CLI flag wiring for this new ConnectSettings.ServerNameOverride field (searching cmd/ shows no references). As-is, the new behavior is not reachable from the shipped sqlcmd commands; please add the flag (and help text) in the Cobra command(s) and map it into ConnectSettings.